One type of PM (permanent magnet) motor is a differential torque type. In that type of motor, torque may be obtained largely as a result of attraction of a PM for the soft magnetic core of the "next" electromagnet; current is applied to the "last" electromagnet to negate backward attraction of the PM for the core of the "last" electromagnet, to thereby release the PM. In common present motors, the current is limited primarily by back EMF, so for a constant moderate torque very high current is drawn at low speed, with the current falling to a moderate level only at a high speed. In applicant's differential torque motor, current for release is limited by coil characteristics in the presence of a high PM magnetic field, and a moderate current can create a moderate torque at both low and high speeds.
Applicant's differential-torque motor has become practical only with the relatively recent (since 1987) availability of high energy-product PMs. Applicant finds that the newer magnetic material, such as Nd-B-Fe (neodymium-boron-iron), has different characteristics than those presented in textbooks for previous PM material. A differential torque motor which took advantage of the characteristics of high energy product PMs to obtain higher starting torque and a large output in a motor of small volume, would be of considerable value. It is noted that a primary application of such motors is in electric powered vehicles, and that while the rotor containing the PMs generally lie within the stator, it is possible to have the rotor rotate around the stator or to use a "pancake" arrangement where the rotor and stator are of the same diameter.